Last modified: 2014-09-23 23:20:15 UTC
Before CC was introduced in MediaWiki, lots of sites are using GFDL and creating free content. However, people without a good knowledge of free culture tend to choose CC-by-nc-sa and have non-free content hosted on their wikis nowadays. This reduces the whole amount of free content we can have in the world.
By the way, ad extensions (such as AdSense) should refuse to work (at least with some warnings) on a nc site. Contributors contributed content with a non-commercial term, so the wiki operator shouldn't use it to get revenue from ads.